Foam formation during microorganism culture is a major concern in food industry and chemical industry where microorganisms are utilized. In many cases, foam is generated when microorganisms are cultured, especially when cultured under aeration and agitation. A foam layer is formed at the upper part of the culture tank and raises problems such as reduction of the working volume in the culture tank, or loss of the culture broth, compositional change of the broth and leakage of the microorganisms due to the outflow of the foam from the upper part of the tank. As such, foam formation causes problems including decrease in the production efficiency and deterioration in quality, and environmental pollution. For this reason, it has been a critical object to suppress foam formation in order to efficiently culture microorganisms for obtaining products. Decrease in the production efficiency due to foam formation has similarly been an object as well in the vinegar production using an acetic acid bacterium.
Therefore, physical and chemical methods have been developed as a defoaming method (see for example, Non-patent documents 1 and 2). As a physical method, there has been known for example, a mechanical method in which a shear force is applied to foam by such as an agitating blade to destroy the foam, a thermal method in which a liquid viscosity is decreased by heating to destabilize the foam, and an electric method in which foam is broken by such as energization, sparking and electric current. All of these methods, however, raise cost for introducing and using the equipments. In addition, defoaming effects brought by these methods have been insufficient.
As a chemical method, a method in which an antifoaming agent is added is exemplified. Compounds such as alcohols, esters, fatty acids and silicon oils are used as a antifoaming agent. However, while a defoaming method using a antifoaming agent is simple, there have been problems caused by some antifoaming agents such as decrease in the oxygen transfer rate which is important for microorganism growth and the material production, inhibition of the microorganism growth, and adverse influence on the isolation and purification steps.
Although the foam formation mechanism during microorganism culture remains largely unknown, some genes and proteins involved in foam formation in eukaryotes have been found. One of those genes is the awa1 gene which is involved in foam formation and found in the yeast (see for example, Non-patent document 3). This gene encodes the glycosylphosphatidylinositol anchor protein which is a protein specific to eukaryotes. This protein is involved in the cell surface hydrophobicity, and foaming ability is suppressed when the gene is disrupted. Further, a protein called hydrophobin which is either hydrophobic or amphipathic was found in fungi, mushrooms, etc. It has been found that the foaming ability is suppressed by disrupting a gene encoding the hydrophobin (see for example, Patent document 1). In prokaryotes, however, it is the current situation that knowledge of genes or proteins involved in foam formation during culture has scarcely been obtained, despite that breeding of bacterial strains with less foam formation had been desired as a novel defoaming means replacing physical or chemical methods.
On the other hand, the presence of an intercellular signal communication system in which transcription of specific genes is controlled depending on the cell density has been recently elucidated in many bacteria. This system is called quorum-sensing system (a control system sensing a cell density) and is involved in the expression control for various functions such as bioluminescence, exoenzyme production, toxic virulence, biofilm formation, and antibiotic production.
Two kinds of proteins are involved in the quorum-sensing system which has been found in many Gram negative bacteria such as Vibrio fischeri (see for example, Non-Patent document 4). The proteins are an acyl homoserine lactone synthase that synthesizes acyl homoserine lactone which is an intracellular signal molecule, and an acyl homoserine lactone receptor-type transcription factor that is a receptor of acyl homoserine lactone and that also functions as a transcription factor. Acyl homoserine lactone produced by an acyl homoserine lactone synthase in a bacterial cell diffuses inside and outside the bacterial cell. As the concentration of acyl homoserine lactone is increased, it forms a complex with the acyl homoserine lactone receptor-type transcription factor in a bacterial cell to control the gene transcription.
The present inventor has already obtained two kinds of genes involved in the quorum-sensing system in an acetic acid bacterium, namely, a gene encoding the acyl homoserine lactone synthase and a gene encoding the acyl homoserine lactone receptor-type transcription factor. In addition, the present inventor has demonstrated that the quorum-sensing system in an acetic acid bacterium is involved in the acetic acid production ability. However, the correlation between the quorum-sensing system and foam formation during microorganism culture has been totally unknown.    Non-Patent Document 1: Principles of Fermentation Technology, Japan Scientific Societies Press, p. 85-86, 1988    Non-Patent Document 2: Technology about foam formation: Use, produce and eliminate, Kogyo Chosakai Publishing, Inc., p. 112-115, 2004    Non-Patent Document 3: Journal of bioscience and bioengineering, Vol. 97, No. 1, pp. 14-18, 2004    Non-Patent Document 4: Bioscience and Industry, Vol. 60, No. 4, pp. 219-224, 2002    Patent Document 1: Published Japanese translation of PCT international publication No. 2003-507056